1. Field of the Invention
The present invention relates to a regulator for a power supply, and in particular, relates to a regulator that compensates for voltage transients at the power supply output.
2. Description of Related Art
A number of advanced microprocessors presently available, such as mobile processors, support power management features with the purpose, for example, of conserving power consumption. In particular, these microprocessors have the ability to dynamically switch between several different operating frequencies. For example, a microprocessor may operate at a high frequency mode for handling instructions and commands as needed, and then switch to a lower frequency mode (e.g., an idle or sleep mode) when the demands on the microprocessor are reduced or cease altogether. Notably, as these microprocessors switch between frequency modes, the operating voltages and current demands of the microprocessor core changes, with the lower frequency modes having lower demands. Accordingly, by switching to a lower frequency mode as performance needs change, these microprocessors are able to reduce their power needs and thereby power consumption, conserving power. Notably, it is not uncommon to operate these advanced microprocessors with thousands of idle modes, for example, realized for every second of operation in order to reduce power consumption.
An advanced microprocessor as described above presents a complicated load to the power supply providing power to the microprocessor core. In particular, as the microprocessor switches between frequency modes, a large differential in power requirements occurs as the core's voltage and current needs change. In addition, the microprocessor core requires a precisely regulated voltage. For example, a power supply must typically maintain its output voltage within a tolerance band of +50 millivolts, for example, of the microprocessor's desired operating voltage for the present frequency mode (hereinafter, this desired operating voltage will also be referred to as a “voltage set-point”). Accordingly, as a microprocessor varies its frequency mode, the power supply must vary its output power while also continuing to maintain a precise output voltage within the voltage tolerance band of the microprocessor.
Referring to FIG. 1 there is shown an example advanced microprocessor 102 interfaced to a power supply 110 that includes a power supply output 112 interfaced to the microprocessor for providing power to the microprocessor core. As illustrated, the power supply may include a voltage regulator 114 capable of generating a large amount of power and providing a precisely regulated output voltage (VCORE) at output 112, as required by microprocessor 102. The voltage regulator may be, for example, a buck voltage regulator that includes a controller 116 with one or more internal drivers (although the drivers may also be external) for driving one or more output stages 117, depending on whether a multi-phase or single-phase configuration is used (for simplicity, only a single phase is shown in FIG. 1). As the microprocessor operates, voltage regulator 114 supplies current and a precisely regulated voltage to the processor, with a majority of the generated energy being stored in output inductors, represented by inductor 122. The output of the inductors is typically transferred to output capacitors, represented by capacitor 124, which are selected to have a low ESR (Equivalent Series Resistance) for generating a highly precise output voltage VCORE to the microprocessor core.
As also illustrated in FIG. 1, advanced microprocessors typically provide multiple voltage indication (VID) output pins 104 (e.g., six VID pins). The microprocessor uses these pins to produce a VID signal, which is a digital representation of the desired voltage set-point the microprocessor core requires at power supply output 112 as the microprocessor switches between frequency modes. Controllers, such as controller 116, that are designed to operate with advanced microprocessors also have corresponding VID input pins 118. In operation, VID output pins 104 of microprocessor 102 are connected to VID input pins 118 of controller 116, thereby allowing the microprocessor to signal the controller with the desired voltage set-point as the microprocessor changes frequency modes. VID input pins 118 may be interfaced to a digital-to-analog converter 120, for example, that converts the digitally represented voltage set-point to an analog reference voltage. Thereafter, the controller controls/drives output stages 117 to maintain this voltage at power supply output 112, as requested by the microprocessor.
As indicated above, advanced microprocessors dynamically switch between different frequency modes in order to conserve power consumption. More specifically, as microprocessor 102 switches from a higher frequency mode to a lower frequency mode, the processor generates a VID signal at VID pins 104 in order to signal controller 116 of the new/reduced voltage set-point, thereby causing controller 116 to drive output stages 117 to reduce the power level at output 112. Similarly, as the microprocessor switches from a lower frequency mode back to a higher frequency mode, the processor signals controller 116 to provide an increased power level at output 112.
Significantly, as a result of microprocessor 102 shifting its power level demands from power supply 110, the power supply is subject to voltage transients at output 112. For example, as microprocessor 102 switches from a high frequency mode to a lower frequency mode and signals a reduced voltage set-point to controller 116, the controller controls output stage 117 to produce less power. As this occurs, inductors 122 are supplied with less current as the output stages adjust, causing the inductors to dissipate their stored energy. This dissipation of stored energy creates a step unload transient in the current. As the inductors discharge, their stored energy is transferred to capacitors 124, which can cause the output voltage of the power supply to momentarily rise, thereby creating a voltage transient. The size of this voltage transient is dependent, for example, on capacitors 124 and on the magnitude of the change in voltage set-point as signaled by microprocessor 102.
Of particular concern is when microprocessor 102 switches from a high frequency mode to a low frequency mode, such as an idle mode, and requests a large change in the voltage set-point. At this time, a high step current transient can be created. Notably, if power supply 110 is not capable of compensating for this large current transient, a large rise in the output voltage at power supply output 112 can result as inductors 122 transfer energy to capacitors 124. As the output voltage rises, an overshoot may occur that may possibly exceed the operating specifications of the microprocessor. For example, microprocessor 102 may have a tolerance band of +50 mV of the processor's voltage set-point and may be able to sustain a maximum overshoot of this size for 25 us, for example. If the voltage regulator does not compensate for the step current transient, the power supply output voltage may exceed such specifications, leading to long term reliability issues or permanent damage to the microprocessor.
For example, a high voltage on the power supply output beyond the specification ranges of the microprocessor can damage semiconductor layers such as oxides, leading to problems with reliability of the semiconductor device. Alternately, a high voltage on the power supply output outside of the specification ranges may permanently damage the processor leading to catastrophic failure. Accordingly, the microprocessor must be protected from over voltage and under voltage conditions within a precise tolerance during the transient states caused by step unload operations.
Notably, the overshoot problem described above also occurs for desktop and server-based microprocessors that operate at a constant frequency mode and a constant operating voltage/voltage set-point. Specifically, even though these microprocessors operate at a constant frequency and voltage, they move between states of high computational workload and idle states. As these microprocessors move to an idle state, the processor's current load decreases, leading to similar current and thereby voltage transients as described above. Again, these microprocessors must be protected from overshoot conditions within a precise tolerance during the transient states caused by step unload operations. (Note that desktop and server-based microprocessors may also provide VID output pins and use these pins to specify to a voltage regulator the processor's desired/constant voltage set-point).
One way for power supply 110 to avoid excessive voltages on power supply output 112 is to provide additional components, such as capacitors. The intent of these additional capacitors is to absorb the excess energy available in the inductors, dampening the transient experienced during the step unload operation while also maintaining a precise power supply output voltage. However, these additional capacitors need to have a low ESR in order to maintain a precise power supply output voltage and are thereby expensive. This additional expense can represent a large cost with respect to an overall system.
Advanced microprocessors, such as microprocessor 102, may also avoid excessive voltages on the power supply output by changing their frequency mode in a step-wise fashion from the present operating frequency to the desired operating frequency. In other words, as indicated above, advanced microprocessors often provide numerous operating frequencies, each with a different operating voltage. The operating voltages for each of the operating frequencies may be separated by 12.5 mV steps, for example. Accordingly, the microprocessor may incrementally drop its operating frequency in step-wise fashion, signaling controller 116 through VID pins 104 to incrementally drop the power supply output voltage (i.e., move the voltage set-point downward in 12.5 mV steps, for example). Between each incremental step downward, the microprocessor may allow for settling time before moving to the next step. By moving in incremental steps with settling times, the step current transients are reduced, making it easier for power supply 110 to compensate for the transients and maintain a precise output voltage at output 112. However, moving between operating frequencies in a step-wise fashion is slow and inefficient.